Now, if the idea that UPDATE and DELETE statements made by other sessions will leak into your view of the database state even within a transaction horrifies you, what you probably want is the other transaction isolation method: serializable.
There are actually two slightly different MVCC modes available in PostgreSQL to help address different requirements here. These reflect two behaviors from the SQL standard, introduced at http://www.postgresql.org/docs/current/static/transaction-iso.html. The problems described there, such as non-repeatable and phantom reads, require using the database's more complicated serializable mode to avoid them, instead of the simpler and default Read Committed transaction model. It's worth reading ...
